A Search Engine is software that searches for data based on some criteria. Although search engines have been around for decades, they were brought to the forefront after the Web exploded onto the scene. Every Web search engine site uses a search engine that it has either developed itself or has purchased from a third party. Search engines can differ dramatically in the way they find and index the material on the Web, and the way they search the indexes from the user's query. Popular search engines include Google.com, search.com, and Bing.com.